Process for the manufacture of fine particulate maddrell{40 s salt

ABSTRACT

Production of fine particulate Maddrell&#39;&#39;s salt by subjecting coarsely granular Maddrell&#39;&#39;s salt to grinding. The ground Maddrell&#39;&#39;s salt is annealed for a period of between 0.5 and 4 hours at a temperature of between 360* and 420* C., and a product with a low fraction of water-soluble matter therein is obtained.

United States Patent Kern et al.

[15] 3,656,096 [451 Apr. 18, 1972 154] PROCESS FOR THE MANUFACTURE OF FINE PARTICULATE MADDRELL'S SALT [72] Inventors: Winfried Kern, l-lurth near Cologne;

Oskar Gehrig, Mannheim; Heinz Harnisch, Lovenich near Cologne, all of Germany [73] Assignees: Knapsack Aktien gesellschaft, Knapsack near Koln; Benckiser-Knapsack Gmbl-l, Ludwigshafen am Rhine, Germany [22] Filed: Nov.4, 1970 [21] Appl.No.: 86,969

30 Foreign Applicatio n Priority Data Nov. 13, 1969 Germany ..P 19 57 063.9

52 US. Cl ..23/106 51 Int. Cl. ..C0lb 25/30 [58] Field of Search ..23/106 Primary Examiner-Oscar R. Vertiz Assistant Examiner-Charles B. Rodman Attorney-Connolly and l-lutz [5 7] ABSTRACT Production of fine particulate Maddrells salt by subjecting coarsely granular Maddrells salt to grinding. The ground Maddrells salt is annealed for a period of between 0.5 and 4 hours at a temperature of between 360 and 420 C., and a product with a low fraction of water-soluble matter therein is obtained.

4 Claims, N0 Drawings PROCESS FOR THE MANUFACTURE OF FINE PARTICULATE MADDRELL 'S SALT of water-soluble substance therein has been described in.

French Patent Nos. 1,549,914 and 1,570,296. On subjecting the products so made to fine grinding, it is found, however,

that the grinding step effects a considerable increase in the initially low water-solubility of the salt.

While the process reported in U.S. Pat. No. 2,356,799 is said to produce Maddrells salt having a water-soluble fraction of less than 4 weight percent therein, it has been shown in Journal of American Society 81 (1959), page 79 that the products so made in fact contain a water-soluble fraction of practically always 5 weight percent.

The present invention now unexpectedly provides a process enabling the increase in the water-solubility of Maddrells salt,

which is occasioned on subjecting the salt to fine grinding, to

be nullified in simple fashion and fine particulate Maddrells salt with a low proportion of water-soluble substance therein to be produced in purposive manner, which process comprises subjecting ground Maddrells salt to annealing treatment for a period of between 0.5 and 4 hours, preferably 1 and 2 hours, at temperatures of between 360 and 420 C., preferably 380 and 400 C.

The annealing step should conveniently be carried out in a heated rotating tube.

It is advantageous, prior to effecting the annealing step, to subject coarse Maddrells salt, which is produced in conven-' tional manner, to grinding so as to obtain particles of which 99 percent have a size of less than 40 microns.

In the process of the present invention, it is even possible to use Maddrells salt having up to 5 weight percent of an acid pyrophosphate contaminant therein, without the soluble fraction of the final product being increased thereby.

The annealing step of the present invention could not be expected to produce the'effect' described above, as the ground salt fails to sinter or cake together or undergo any other coarsening of its particles, during the annealing step.

The annealing step does not simply effect full dehydration of the product. What it effects is an inactivation of the products surface with the result that the ground salt ceases to provide further water, during the annealing treatment. The products treated as described in the present invention have a watensoluble fraction of less than 3 weight percent therein.

In French Patent 1,570,296 it has been reported that the narrow temperature range, within which the conversion of acid sodium phosphate to Maddrells salt takes place within commercially attractive periods, is one of the particular difficulties that are encountered in the production of Maddrells salt, together with the need to maintain a given steam partial In other words, in the process of the present invention it is no longer imperative that the Nal-IJO, be completely dehydrated to Maddrells salt in a single step, as in earlier processes. As results therefrom, a considerably enlarged margin is available for the maximum dehydration temperature, and the process is technically easier to carry out.

EXAMPLE 1 A 5 gram specimen of Nal-LPO. was dehydrated in the manner described in French Patent No. 1,570,296 to Maddrells salt, and the salt was crushed in a procelain mortar. The specimen was found to have a solubilityv of 0.72 percent. It was then ground for several hours in an agate mortar until it could be passed through a 40 micron sieve, without residue. After that treatment, the solubility was 11.1 percent. The specimen so treated was: annealed for minutes and then had a solubility as low as 1.5 percent.

EXAMPLE 2 ZOkg/hr of finely ground Maddrells salt (99 percent 40 microns) were conveyed through anelectrically heated rotating tube, which was 2.50 meters long and 250 millimeters wide. The product reached a temperature of 380 C.

The Maddrells salt so treated contained the following proportions of soluble matter:

Prior to annealing treatment Afier annealing treatment 5.46 weight percent 2.38 weight percent 4.16 weight percent 2.43 weight percent 3.99 weight percent 2.60 weight percent 2.56 weight percent 2.04 weight percent pressure value. While the above conversion is theoretically initiated at temperatures above 250 C., the fact remains that temperatures of about 400 C. and more are required to be produced to make the reaction proceed at a speed satisfactory for commercial purposes. On the other hand, however, Maddrells salt is known to commence conversion to sodium trimetaphosphate, at temperatures around 450 C.

This problem is obviated by the annealing step of the present invention, which even enables up to 5 weight percent of pyrophosphate, based on the weight of Maddrells salt used, to be completely transformed into difficultly soluble Maddrells salt, and this again in the absence of any caking phenomena in the rotating tube, or coarsening of the particles of the final product.

99 Weight percent of the product had a particle size of less than 40 microns, prior to and after the annealing treatment. EXAMPLE 3 A specimen of Maddrells salt, which was prepared in the manner described in French Patent 1,570,296, was found to have a solubility of 5.35 grams of P 0 based on 100 grams of substance. The specimen contained acid pyrophosphate in a proportion of n 3.0 grams/100 grams, based on P 0 The specimen was annealed for 30 minutes at 380 400 C. and then had a solubility of only 1.69 grams of P 0 per 100 grams of substance. The proportion of pyrophosphate initially present in the feed specimen was found to have been transformed into Maddrells salt, during the annealing step.

A comparison of the P O -fractions soluble prior to and after the annealing step confirmed the statement already made above that the decrease of solubility effected during the annealing is in no way solely occasioned by the removal of water of constitution.

Solu Theor.

bility Pyrophossolu- Actual solu- Pyroprior to phate probility bility phosannealing portion phate step 2; 1 5! 2; P205, 3 r d s P2051100 3 We claim:

1. A process for manufacturing fine particulate Maddrells salt having a low water-solubility which comprises grinding coarsely granular Maddrells salt containing up to 5 weight percent of an acid pyrophosphate contaminant to particles of which 99 percent have a size of less than 40 microns, annealing the said ground Maddrells salt particles for a period of between 0.5 to 4 hours at temperatures between 360 and 420 C. with the resultant formation of a fine particulate Maddrells salt having a water-soluble fraction of less than 3 weight percent therein.

2. The process as claimed in claim 1, wherein the annealing treatment is effected at a temperature of between 380 and 400 C.

3. The process as claimed in claim 1, wherein the ground Maddrells salt is annealed for a period of between 1 and 2 hours.

4. The process as claimed in claim 1, wherein the annealing treatment is effected inside a heated rotating tube. 

2. The process as claimed in claim 1, wherein the annealing treatment is effected at a temperature of between 380* and 400* C.
 3. The process as claimed in claim 1, wherein the ground Maddrell''s salt is annealed for a period of between 1 and 2 hours.
 4. The process as claimed in claim 1, wherein the annealing treatment is effected inside a heated rotating tube. 